The present invention relates generally to devices for testing the adequacy of air removal in a sterilization apparatus, and in particular, a prevacuum sterilization apparatus utilized for sterilizing medical equipment. Even more particularly, the present invention relates to a test pack having a steam reactive chemical indicator to determine the efficacy of air removal in a sterilization apparatus. The present invention is particularly useful, although not exclusively useful, for the testing of air removal in prevacuum sterilization equipment used for the steam sterilization of hospital medical equipment.
The sterilization of medical equipment by exposure to sterilization gas such as steam is typically accomplished by using an autoclave. Normally the equipment to be sterilized is placed into the autoclave and a vacuum may or may not be drawn depending on the particular procedure being followed. The sterilization medium, steam, is then introduced into the autoclave to permeate the equipment and sterilize it.
In the gravity displacement steam sterilizer, a sterilizer chamber, with objects to be sterilized within, is subjected to steam for a predetermined period of time. The steam inlet to the camber is typically located in the middle of the rear wall of the chamber. Because steam is lighter than air, it will accordingly rise towards the top of the chamber, displacing the air towards the bottom of the chamber and out the chamber drain. A temperature sensor and a thermostat are provided in the exhaust line to detect when the temperature rises sufficiently, indicating that steam is exiting the chamber, and that, accordingly, air has been completely displaced by the steam.
The above described sterilization apparatus is called a gravity displacement steam sterilizer because gravity and steam under pressure are relied on to cause the lighter steam to displace the heavier air towards the drain in the bottom of the chamber. Because the chamber is constantly under positive pressure, it is impossible for air to enter. The entry of steam under pressure and the operation of the thermostat in the drain ensure total air removal.
In another form of sterilizer, called a prevacuum sterilizer, shown schematically in FIG. 10, the vacuum pump 100 is usually first operated before steam is supplied into the sterilizing chamber 102 to evacuate the air in the chamber. Once air has been partially evacuated, after operation of the pump for a sufficient time, steam is introduced into the chamber via steam inlet 104 until a specified pressure is reached. A series of vacuum pulses followed by introductions of steam are used depending on the model of sterilizer. These steam/vacuum pulses are designed to remove virtually all the air from the chamber and the packs within. After the final vacuum, steam is introduced into the chamber until the correct sterilization temperature/pressure is attained. During the vacuum phase, air reentry is possible because the chamber is under a negative pressure. Air can be drawn into the chamber via leaks in the door gasket or other areas. In these instances, the residual air in the chamber and packs can act as a deterrent to steam penetration into the packs to be sterilized.
According to recommended procedures, steam sterilization equipment in hospitals and other health care facilities needs to be periodically tested to ensure the sterilization procedure is efficacious. In one test, resistant bacterial spores are subjected to the sterilization cycle and subsequently observations are made as to whether they have remained viable. To ensure the sterilization process is adequate, the spores are placed into a specified challenge pack. This pack is designed to challenge penetration by steam as much as or better than the challenge provided by standard hospital packs and packaging. The test is carried out in a normally loaded sterilizer under normal conditions. It is a true test of the sterilizer""s ability to sterilize. This test is known as a biological test.
Several procedures have been proposed to test the sterilization efficacy of steam sterilization equipment. Typical of these and perhaps the best known and most widely accepted, is the procedure recommended by the Association for the Advancement of Medical Instrumentation (AAMI) as ANSI/AAMI ST46-1993 (xe2x80x9cGood Hospital Practice: Steam Sterilization ad Sterility Assurancexe2x80x9d). According to the AAMI recommended practice, 16 freshly laundered all cotton towels are folded by hospital personnel and stacked to construct a challenge test pack approximately 6 inches high. The biological indicator is embedded into the center of this challenge pack. This pack is then subjected to the sterilization cycle as part of a routine sterilization load.
Although apparently efficacious for its intended purpose, the construction of a biological challenge pack according to the AAMI procedure is labor intensive and the resulting pack is relatively bulky.
Applicant""s prior U.S. Pat. No. 5,345,971 provides a biological challenge type test pack which obviates the problems inherent in the AAMI procedure by providing a disposable biological challenge pack which has the same performance characteristics as the referenced AAMI towel challenge pack. It is suitable for use as a sterilization test pack for hospital sterilizers.
A different type of test, which employs a different test pack is also recommended for the routine testing of hospital sterilizers. This test is limited to prevacuum sterilizers only. The aim in a prevacuum sterilizer is to remove virtually all air prior to the final introduction of steam for sterilization into the chamber. A test pack known as a Bowie-Dick test pack has been used in such prevacuum tests in the past. In particular, the test pack 112 of FIG. 10 is used to determine the adequacy of the air removal from inside the test pack and replacement of the air by steam. Unlike the biological test, this is not a test of sterilization. It is solely a test of air removal. It is carried out under conditions where the test pack is the only pack in the sterilizer, whereas the biological challenge pack is used in a full load.
In these Bowie-Dick types of test packs, the pack includes a sheet of paper printed with a steam-reactive ink which will change color in the presence of steam under pressure, but will not change color in the presence of air heated to the same degree as steam. The pack is placed into a prevacuum sterilizer, and a cycle is run. The test sheet is removed from the pack and examined to determine if air was removed and replaced by steam. If the air removal has been adequate, the indicator ink will have uniformly changed color throughout the test sheet due to the even presence of steam in the pack and no residual air entrapped in the pack. If the air removal process was in some way defective, air would be trapped in the center of the pack, and its presence would prevent steam from evenly contacting the test sheet and steam reactive ink. In this case, the reactive ink throughout the test sheet would exhibit an uneven color development. Since it is the only pack in the sterilizer, the Bowie-Dick pack provides a sensitive detector of the presence of residual air. If air is not detected by this pack, it can be assumed that air will be adequately removed from packs and will not reenter them during a sterilization run in a fully loaded sterilizer.
In order to assist personnel in determining if color development was correct, and thus air removal was complete, a sample of a correctly exposed test sheet or simulation as well as sheets showing marginal as well as major air entrapment are usually provided for comparison purposes.
An example of a prior art Bowie-Dick test pack is shown in FIG. 9. The figure shows a number of stacked towels about 10-11 inches in height, about twelve inches in length and about 9 inches in width with a Bowie-Dick test sheet or crossed tape disposed at approximately the center of the height dimension. The test sheet has deposited ink or a tape thereon which changes color in the presence of steam. The assembled stack 114 is then wrapped in an air and steam permeable overwrap 116 which is folded about the stack. This arrangement is quite bulky and time consuming to assemble, and clearly does not provide a quick, simple and time-saving vacuum removal testing device. It has substantial differences in construction and use as compared to a biological challenge pack.
The biological test pack as shown in U.S. Pat. No. 5,435,971, although useful as a test of sterilization, is not useful as an air removal test in a prevacuum sterilizer, since it is incapable of providing an evaluation of whether air has been sufficiently evacuated. This is because it is not designed for this purpose, but instead for the purpose of testing sterilization efficacy. It has been shown to be comparable in performance to the AAMI recommended biological challenge pack of 16 towels 6 inches high.
U.S. Pat. Nos. 4,579,715, 4,576,795 and 4,692,307 to Bruso show prevacuum sterilizer test packs. U.S. Pat. No. 4,596,696 to Scoville Jr. shows another form of prevacuum sterilizer test pack. Each of these patents disclose a test pack comprising a chemical indicator which is sandwiched between two pluralities of sheets of porous material. In the Bruso devices, the porous material, both on the top and bottom, is covered by non-porous or impermeable layers. These test packs substantially prevent gas movement through the top and bottom surfaces and only allow gas to flow substantially through the sides of the porous sheets of the test pack. For example, each of the three Bruso vacuum test pack patents show a substantially impermeable top layer 16 (and bottom layer 30) which inhibits gas flow through the top (and bottom). This leads to problems with respect to precluding gas flow to and from the interior through the top surface, thereby emphasizing the flow of gas to and from the interior along the edges and intermediate the porous sheets of paper. This contributes to making the test pack highly sensitive to the degree of tightness with which an overwrap material is applied thereto by affecting the ability of the gas blocked by the gas impermeable layer or layers on the top and bottom to reach the interior.
The devices of the Bruso patents, because of their construction allowing gas to ingress and egress through the sides, have been found to be under-sensitive, i.e., residual air present in the test pack at places other than of the chemical indicator is not sensed, thus leading to false and unreliable indications of air removal.
In the Scoville, Jr. patent, the entire test pack is housed in a box made of a paperboard laminated with a polyester film lamination. This makes the box relatively impermeable to steam. In order to allow steam to permeate into the box and to provide a challenge to steam sterilization, holes or slots are provided in the box to allow air egress and steam ingress.
The device of the Scoville, Jr. reference presents significant barriers to the egress of the air and the entry of the steam. The paperboard box in which the pieces of porous material and chemical indicator are housed is substantially air and gas impermeable due to the lamination. The air can only leave the box, and steam enter the box, primarily through the openings provided in the box as well as through cracks at the ends of the box where the box closure flaps are folded together. The Scoville, Jr. device thus is over-sensitive, i.e., it presents what is believed to be an unnecessary degree of air removal challenge and may provide false indications of lack of air removal when in fact air removal has been accomplished.
All previous test pack designs have been based upon packs of symmetrical construction. Air egress and steam entry are inhibited by areas of relatively gas impermeable layers either at the top and bottom or as a total overwrap. This presents a disadvantage in that if a pack is found to be either under-sensitive or over-sensitive during the manufacturing process, the manufacturer must either add or subtract sheets from the stacks which comprise the top and bottom pluralities of paper, vary the porosity of the papers in the stacks, or adjust the chemistry of the ink used on the sheet. This is a time consuming and difficult process.
The biological test pack of the Dyckman ""971 patent provides a first plurality of porous sheets having apertures therein for housing a biological indicator and second and third pluralities of unapertured porous sheets on either side of the first plurality of sheets, the plurality of sheets being housed in a tray formed of gas inhibiting material. The tray is open at the top and the tray with the three pluralities of sheets are enclosed in an overwrap covering material or an organizer box. The covering material or organizer box is substantially air and steam permeable while the tray is substantially impermeable to air and steam. This causes the air to egress and the steam to enter essentially through the open-tray planar top of the test pack. The test pack of the ""971 patent is not unduly sensitive to the tightness with which the overwrap material is applied thereto because gas and air movement is not primarily through the sides of the test pack, but instead through the top.
The test pack of the ""971 patent, being a sterilization biological test pack, is primarily directed to a gravity displacement type steam sterilization apparatus. It is incapable of evaluating the degree of air removal and replacement of air by sterilizing steam. Nor have test packs of its construction been employed in prevacuum sterilizers for testing adequacy of air removal, as these test packs are not designed for this purpose.
However, there is a need for a test pack which is simple to use, reliable, cost-effective and time-saving and which provides an accurate gauge of the degree of air removal in a prevacuum sterilizer. All of the prior art devices for prevacuum sterilizer air removal testing suffer from various disadvantages, as discussed above, and thus, the industry needs a better test pack for this purpose.
The inventor has discovered, contrary to the accepted techniques for making air removal test packs, that a prevacuum sterilizer test pack can be constructed using a substantially impermeable tray similar to the tray of the biological test pack of the ""971 patent, and which gives reliable indications of adequacy of air removal.
The invention employs a non symmetrical assembly in which five of the six planar sides of the assembly are disposed next to a relatively gas impermeable laminated tray. This non symmetrical construction causes air egress and steam entry mainly through the top of the assembly.
It is an object of the present invention to provide a test pack for determining adequacy of air removal in pre-vacuum gas sterilization equipment.
It is yet still a further object of the present invention to provide a test pack which is useful in prevacuum sterilizers where the sterilizing gas is steam.
Yet still a further object of the present invention is to provide a test pack which is simple, reliable, cost-effective and time-saving to use and which provides good results.
Yet still a further object of the present invention is to provide a test pack for testing the adequacy of air removal in a prevacuum steam sterilizer which is not overly or under-sensitive to the degree of air removal.
Yet still a further object is to provide such an air removal test pack which can be customized to present different degrees of air removal challenge.
The above and other objects of the present invention are achieved by a test pack for testing the adequacy of air removal in a prevacuum sterilizer, comprising a first plurality of planar sheets of substantially porous material disposed to form a stack; a second plurality of planar sheets of substantially porous material disposed to form a stack; an indicator adapted to provide an indication of the removal of air within the test pack and the replacement of the air by a sterilizing gas; the indicator being disposed between said first and second pluralities of planar sheets; a tray formed of a gas inhibiting material, the tray having a base coextensive and aligned with an exposed side of one of said first and second pluralities of sheets and four side walls transverse to said base together substantially covering the exposed edges of said first and second pluralities of sheets having said indicator sandwiched between said first and second pluralities of sheets; the tray and the first and second pluralities of sheets with the indicator disposed therebetween defining an assembly, the tray being open on one side opposite the base such that a surface of one of said first and second pluralities of sheets is not covered; and an organizer disposed all about the periphery of the assembly to maintain the sheets and the tray in a predetermined spatial relationship while permitting the passage of sterilizing gas and air to and from the assembly.